1. Field of the Invention
The present invention pertains to methods and instruments for measuring inertial and gravitational forces. More particularly, this invention is directed to methods and apparatus for reducing accelerometer error in the measurement of acceleration and/or gravity.
2. Description of the Prior Art
Accelerometer bias uncertainty is one of the fundamental error sources in inertial navigation systems. Such error represents the portion of the accelerometer bias that varies with time, temperature, thermal gradient, magnetic field, radiation, mounting force, mechanical shock and vibration, pressure, humidity, etc. Bias uncertainties in excess of 100 ug, which exceed the bounds of acceptable error in numerous present-day applications, are typical of present-day accelerometers.
Efforts to lower this source of error in pendulous-type systems have involved the compromise of significant instrument parameters. Such attempts include designs that lower the ratio of the suspension spring rate to proofmass pendulosity. This general approach leads to lower shock and vibration capabilities and to reduced structural natural frequencies which, in turn, limit the bandwidth of the servo, lower midband gain and frequency, and cause the minimum stiffness of the servo to coincide with the resonant frequency of the system isolators.(The latter effect limits the maximum dynamic g capability of the accelerometer.) Additionally, lowered structural natural frequencies can coincide with gyro-induced vibrations, requiring the "tuning" of the accelerometer's suspension to minimize rectification effects.
Other attempts have centered upon correction of the portion of uncertainty that results from hysteresis and suspension elastic after-effect. Such proposed solutions have proven costly, involving precision manufacture and instrument adjustment. Thus, current methods for minimizing bias uncertainty have generally increased costs significantly, reduced system dynamic performance and lowered environmental capabilities.